LAVES PHASE ALLOYS FOR HIGH TEMPERATURE APPLICATIONS

Ternary Laves phases with transition metals and aluminium were overviewed with respect to their use for structural high temperature applications. The relation between constitution, phase stability, crystal structure and basic mechanical properties was discussed. The crystal structure-hexagonal C14 structure or cubic C15 structure-is a function of atomic radii and valence electron concentration. A strong positive correlation of basic mechanical properties-hardness, yield stress-with the sublimation energy of the phases was found. The brittle to ductile transition temperature was observed at about 60% of the melting temperature. The regarded ternary Laves phases allow alloying to form stable equilibria with the less hard and brittle B2 aluminides. Advantageous examples are alloys of type NbNiAl NiAl, TaNiAl NiAl, and TaFeAl FeAl. Multiphase NiAl Ta Cr alloys are regarded as promising for structural high temperature applications and have been selected for a materials development which aims at applications in gas turbines at temperatures above those of Ni base superalloys, such alloy can be prepared by ingot metallurgy and powder metallurgy and hot forming is possible. Characteristic data were presented with respect to mechanical behaviour. The alloys showed a high thermoshock resistance in spite of the comparatively high brittle to ductile transition temperature. The prospects of alloy development were discussed.

Phase Transformation and Thermal Stability of Aged Ti-Ni-Hf High Temperature Shape Memory Alloys

The use of Ni-rich TiNiHf alloys as high temperature shape memory alloys （SMAs） through aging has been presented. For Ni-rich Ti80-xNixHf20 alloys, their phase transformation temperatures are averagely increased more than 100 K by aging at 823 K for 2 h. Especially for the alloys with Ni-content of 50.4 at. pct and 50.6 at. pct, their martensitic transformation start temperatures （Ms） are more than 473 K after aging. TEM observation confirmed that some fine particles precipitate from the matrix during aging. The aged Ni-rich TiNiHf SMAs show the better thermal stability of phase transformation temperatures than the solutiontreated TiNiHf alloys. The fine particles precipitated during aging should be responsible for the increase of phase transformation temperatures and its high stability.

In-Situ High Temperature X-Ray Diffraction Study of Structure and Phase Transformation of Nd-FePt Alloys

The structure and disorder-order transformation of NdxFe60.5-x Pt39.5（x = 0, 0.5, 1.0, 1.5） alloys were investigated in situ by high temperature X-ray diffraction. The results show that the lattice parameter a of disordered γ phase （FCC, Al structure type） and the lattice parameter ratio c/a of ordered γ1 phase （FCT, L10 structure type） increase linearly with increasing Nd concentration, whereas the c/a ratio decreases with increasing temperature. The transition temperature from ordered FCT to disordered FCC decreases with increasing Nd concentration, but for alloys quenched rapidly from γ phase region into ice-water it increases with increasing Nd.

Effect of high magnetic field on phase transformation temperature and microstructure of Fe-based Alloys

The effect of a high magnetic field up to 30T on phase transformation temperature and microstructure of Fe-based alloys has been reviewed. A high magnetic field accelerates ferrite transformation, changes the morphology of the transformed microstructures and increases the A3 and A1 temperature. In a magnetic field of 30T, the A1 temperature increases by about 37.1℃ for Fe-0.8C, the A3 temperature for pure Fe increases by about 33.1℃. The measured transformation temperature data are not consistent with calculation results using Weiss molecular field theory. Ferrite grains are elongated and aligned along the direction of magnetic field in Fe-0.4C and Fe-0.6C alloys by ferrite transformation, but elongated and aligned structure was not found in pure Fe, Fe-0.05C alloy and Fe-1.5Mn-0.11C-0.1V alloy.

Phase-Transitions at High, Very High, and Very Low Temperatures upon Nano-Indentations: Onset Forces and Transition Energies

This paper describes the phase-transition energies from published loading curves on the basis of the physically deduced FN = k-h3/2 law that does not violate the energy law by assuming h2 instead, as still do ISO-ASTM 14,577 standards. This law is valid for all materials and all “one-point indentation” temperatures. It detects initial surface effects and phase-transition kink-unsteadiness. Why is that important? The mechanically induced phase-transitions form polymorph interfaces with increased risk of crash nucleation for example at the pickle forks of airliners. After our published crashing risk, as nucleated within microscopic polymorph-interfaces via pre-cracks, had finally appeared (we presented microscopic images (5000×) from a model system), 550 airliners were all at once grounded for 18 months due to such microscopic pre-cracks at their pickle forks (connection device for wing to body). These pre-cracks at phase-transition interfaces were previously not complained at the (semi)yearlycheckups of all airliners. But materials with higher compliance against phase- transitions must be developed for everybody’s safety, most easily by checking with nanoindentations, using their physically correct analyses. Unfortunately, non-physical analyses, as based on the after all incredible exponent 2 on h for the FN versus h loading curve are still enforced by ISO-ASTM standards that cannot detect phase-transitions. These standards propagate that all of the force, as applied to the penetrating cone or pyramid shall be used for the depth formation, but not also in part for the pressure to the indenter environment. However, the remaining part of pressure (that was not consumed for migrations, etc.) is always used for the elastic modulus detection routine. That severely violates the energy-law! Furthermore, the now physically analyzed published loading curves contain the phase-transition onsets and energies information, because these old-fashioned authors innocently (?) published (of course correct) experimental loading curves. These follow as ever the physically deduced FN = k-h3/2 relation that does not violate the energy law. Nevertheless, the old-fashioned authors stubbornly assume h2instead of h3/2 as still do ISO-ASTM 14,577 standards according to an Oliver-Pharr publication of 1992 and textbooks. The present work contributes to understanding the temperature dependence of phase-transitions under mechanical load, not only for aviation and space flights, which is important. The physical calculations use exclusively regressions and pure algebra (no iterations, no fittings, and no simulations) in a series of straightforward steps by correcting for unavoidable initial effects from the axis cuts of the linear branches from the above equation exhibiting sharp kink unsteadiness at the onset of phase transitions. The test loading curves are from Molybdenum and Al 7075 alloy. The valid published loading curves strictly follow the FN = k-h3/2 relation. Full applied work, conversion work, and conversion work per depth unit show reliable overall comparable order of magnitude values at temperature increase by 150°C (Al 7075) and 980°C (Mo) when also considering different physical hardnesses and penetration depths. It turns out how much the normalized endothermic phase-transition energy decreases upon temperature increase. For the only known 1000°C indentation we provide reason that the presented loading curves changes are only to a minor degree caused by the thermal expansion. The results with Al 7075 up to 170°C are successfully compared. Al 7075 alloy is also checked by indentation with liquid nitrogen cooling (77 K). It gives two endothermic and one very prominent exothermic phase transition with particularly high normalized phase-transition energy. This indentation loading curve at liquid nitrogen temperature reveals epochal novelties. The energy requiring endothermic phase transitions (already seen at 20°C and above) at 77 K is shortly after the start of the second polymorph (sharply at 19.53 N loading force) followed by a strongly exothermic phase-transition by producing (that is losing) energy-content. Both processes at 77 K are totally unexpected. The produced energy per depth unit is much higher energy than the one required for the previous endothermic conversions. This exothermic phase-transition profits from the inability to provide further energy for the formation of the third polymorph as endothermic obtained at 70°C and above. That is only possible because the very cold crystal can no longer support endothermic events but supports exothermic ones. Both endothermic and exothermic phase-transitions at 77 K under load are unprecedented and were not expected before. While the energetic support at 77 K for endothermic processes under mechanical load is unusual but still understandable (there are also further means to produce lower temperatures). But strongly exothermicphase-transition under mechanical load for the production of new modification with negative energy content (less than the energy content of the ambient polymorph) at very low temperature is an epochal event here on earth. It leads to new global thinking and promises important new applications. The energy content of strongly exothermic transformed material is less than the thermodynamic standard zero energy-content on earth. And it can only be reached when there is no possibility left to produce an endothermic phase-transition. Such less than zero-energy-content materials should be isolated, using appropriate equipment. Their properties must be investigated by chemists, crystallographers, and physicists for cosmological reasons. It could be that such materials will require cooling despite their low energy content (higher stability!) and not survive at ambient temperatures and pressures on earth, but only because we do not know of such negative-energy-content materials with our arbitrary thermodynamic standard zeros on earth. At first one will have to study how far we can go up with temperature for keeping them stable. Thus, the apparently never before considered unprecedented result opens up new thinking for the search of new polymorphs that can, of course, not be reached by heating. Various further applications including cosmology and space flight explorations are profiting from it.

High-temperature electrochemical performance and phase composition of Ti_(0.7) Zr_(0.5) V_(0.2) Mn_(1.8-x)Ni_x hydrogen storage electrode alloys

The rapid development of electric vehicles demands the development of high performance nickel metal hydride battery that is able to endure high temperature. The discharge properties of Ti 0.7 Zr 0.5 V 0.2 Mn 1.8- x Ni x ( x =0.4, 0.8, 1.1, 1.4, 1.7)hydrogen storage alloys was investigated and its phase composition was analyzed using X ray diffraction. The results show that the cycling life was improved as the content of nickel increases. When x =0.4, 0.8, 1.1 and 1.4, the main phase is MgZn 2 type C14 Laves phase and the second one is cubic TiNi phase. When x =1.7, the Laves phase structure disappears. EDAS analysis shows that the increase of nickel content is effective in suppressing the dissolution of vanadium component in alloys. [

Development of high-strength magnesium alloys with excellent ignition-proof performance based on the oxidation and ignition mechanisms: A review

High reactivity and ease of ignition are the major obstacles for the application of Mg alloys in aerospace.Thus,the ignition mechanisms of Mg alloys should be investigated systematically,which can guide the ignition-proof alloy design.This article concludes the factors influencing the ignition resistance of Mg alloys from oxide film and substrate microstructure,and also the mechanisms of alloying elements improving the ignition resistance.The low strength is another reason restricting the development of Mg alloys.Therefore,at the last section,Mg alloys with the combination of high strength and good ignition-proof performance are summarized,including Mg-Al-Ca based alloys,SEN(Mg-Al-Zn-Ca-Y)alloys as well as Mg-Y and Mg-Gd based alloys.Besides,the shortages and the future focus of theses alloys are also reviewed.The aim of this article is to promote the understanding of oxidation and ignition mechanisms of Mg alloys and to provide reference for the development of Mg alloys with high strength and excellent ignition-proof performance at the same time.

Mechanical properties and electronic structures of MgCu_2, Mg_2Ca and MgZn_2 Laves phases by first principles calculations

Mechanical properties and electronic structure of MgCu2, Mg2 Ca and MgZn2 phases were investigated by means of first principles calculations from CASTEP program based on density functional theory（DFT）. The calculated lattice parameters are in good agreement with the experimental and literature values. The calculated heat of formation and cohesive energies showed that MgCu2 has the strongest alloying ability and structural stability. Elastic constants of MgCu2, Mg2 Ca and MgZn2 were calculated, and the bulk moduli, shear moduli, elastic moduli and Poisson ratio were derived. The calculated results show that MgCu2, Mg2 Ca and MgZn2 are all ductile phases. Among the three phases, MgCu2 has the strongest stiffness and the plasticity of MgZn2 phase is the best. Melting points of the three phases were predicted using cohesive energy and elastic constants. Density of states（DOS）, Mulliken population, electron occupation number and charge density difference were discussed. Finally, Debye temperature was calculated and discussed.

High-Temperature Stability of Mg-1Al-12Y Alloy Containing LPSO Phase and Mechanism of Its Portevin-Le Chatelier(PLC)Effect

In this study,the high-temperature stability and the generation mechanism of the Portevin-Le Chatelier(PLC)effect in solid-solution Mg-1Al-12Y alloy with different heat treatment processes were investigated by adjusting the content of long-period stacking ordered(LPSO)phases.It was found that the content of LPSO phases in the alloys differed the most after heat treatment at 530℃for 16 h and 24 h,with values of 13.56%and 3.93%respectively.Subsequently,high-temperature tensile experiments were conducted on these two alloys at temperatures of 150℃,200℃,250℃,and 300℃.The results showed that both alloys exhibited the PLC effect at temperatures ranging from 150 to 250℃.However,at a temperature 300℃,only the alloy with a greater concentration of LPSO phases exhibited the PLC effect,whereas the alloy with a lower proportion of LPSO phases did not exhibit this phenomenon.Additionally,both alloys exhibited remarkable high-temperature stability,with the alloy containing a greater percentage of LPSO phases also demonstrating superior strength.The underlying mechanism for this phenomenon lies in the exceptional high-temperature stability exhibited by the second phase within the alloy.Furthermore,the LPSO phase effectively obstructs the movement of dislocations,and it also undergoing kinking to facilitate plastic deformation of the alloy.The results indicate that the PLC effect can be suppressed by reducing dislocation pile-up at grain boundaries,which leads to a decrease in alloy plasticity but an increase in strength.The presence of the PLC effect in the WA121 alloy is attributed to the abundant dispersed second phase within the alloy,which initially hinders the movement of dislocations,leading to an increase in stress,and subsequently releases the dislocations,allowing them to continue their movement and thereby reducing in stress.

Effect of Ca on microstructure and high temperature creep properties of AM60-1Ce alloy

A series of AM60-1Ce-xCa(x=0, 0.5, 1.5, 2.5) magnesium alloys were prepared by gravity casting method and analyzed by means of XRD, DSC and SEM. The effects of Ca on normal temperature mechanical properties and high temperature creep behavior of alloys were characterized by tensile and constant creep test.Microstructure analysis indicated that Ca was preferentially combined with Al in the alloy to form the high melting point Al_2Ca phase at the grain boundary. The addition of Ca can refine the crystal grains and reduces the content of β-Mg_(17)Al_(12) phase. With the increase of Ca content in the alloy, Al_2Ca phases at the grain boundary gradually changed to the network of lamellar structure, and replaced the β-Mg_(17)Al_(12) phase as the main strengthening phase gradually. The creep resistance of the alloy continuously increases because the high-temperature stable phase Al_2Ca firmly nailed at grain boundaries impedes the sliding of grain boundaries. However, when the addition of Ca was more than 1.5%, mechanical properties of the alloy started to decrease, which was probably due to the large amount of irregularly shaped Al_2Ca phases at the grain boundary. Experimental results show that the optimal addition amount of Ca is 1.5 wt.%.

Tribological property and high temperature oxidation behavior of Ni-based alloy

Ni-Cr based alloys were prepared by hot-pressing the mixture of strengthening phases Mo, Al, Ti, and lubricant phase MoS2. The hardness, tribological properties as well as the high temperature oxidation properties were evaluated, The results show that the strengthening phases can improve the mechanical properties of Ni-Cr based alloy obviously, and the wear and friction properties of Ni-based alloy with strengthening phase can be improved. Its friction coefficient and wear rate rubbed with Al2O3 ceramic disk are about 0.4 and 10 -14m3/（N·m）, respectively, and the oxidation process is mainly affected by Cr2O3.

Oxidation behavior of three-phase Cu-25Ni-30Cr alloy at 700-800℃ under high oxygen pressures

The thermogravimetric analysis of a ternary Cu-25Ni-30Cr alloy prepared by conventional casting was performed in 0.1MPa pure O2 at 700-800℃. The results show that the alloy is composed of three phases, where the phase with the largest copper and lowest chromium content forms the matrix, while the other two, much richer in chromium, form a dispersion of isolated particles. At variance with another three-phase Cu-20Ni-20Cr alloy, which forms complex scales containing the oxides of the various components and double oxides plus an irregular region composed of alloy and oxides, the present alloy can form a very irregular but continuous chromia layer at the base of the mixed internal region, producing a gradual decrease of the oxidation rate down to very low values. A larger chromium content needed to form chromia layer for a ternary three-phase alloy is attributed to the limitations to the diffusion of the alloy components in the metal substrate imposed by their multiphase nature.

Effect of grain size on high-temperature oxidation behavior of Cu-80Ni alloy

The thermogravimetric analysis of binary Cu 80Ni alloys prepared respectively by conventional casting(CA) and mechanical alloying(MA) techniques and presenting widely different grain sizes was performed at 800 ℃ in air in order to study the effect of grain size change on the oxidation behavior of a solid solution alloy. The results show that the kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law and usually are not composed of a single stage. Mixed scales were produced on the CACu 80Ni alloy surface, which consists of a mixture of copper and nickel oxides. However, oxide scale for MACu 80Ni alloy is mainly composed of a thick compact and continuous inner layer of nickel oxide. The reduction in the alloy grain size speeds up the diffusion of the more reactive component nickel from the alloy to alloy/oxide scale interface and completes the transition from a mixed scale to continuous scale of nickel oxide.

ADVANCED NICKEL-BASED AND NICKEL-IRON-BASED SUPERALLOYS FOR CIVIL ENGINEERING APPLICATIONS

The use of high-temperature materials is especially important in power station construction, heating systems engineering, furnace industry, chemical and petrochemical industry, waste incineration plants, coal gasification plants and for flying gas turbines in civil and military aircrafts and helicopters. Particularly in recent years, the development of new processes and the drive to improve the economics of existing processes have increased the requirements significantly so that it is necessary to change from well-proven materials to new alloys. Hitherto, heat resistant ferritic steels sufficed in conventional power station constructions for temperatures up to 550℃ newly developed ferritic/martensitic steels provide sufficient strength up to about 600 - 620℃. In new processes, e.g. fluidized-bed combustion of coal, process temperatures up to 900℃ occur. However, this is not the upper limit, since in combustion engines, e.g. gas turbines. Material temperatures up to 1100℃ are reached locally. Similar development trends can also be identified in the petrochemical industry and in the heat treatment and furnace engineering. The advance to ever higher material temperatures now not only has the consequence of having to use materials with enhanced high-strength properties, considerable attention now also has to be given to their chemical stability in corrosive media. Therefore not only examples of the use of high-temperature alloys for practical applications will be given but also be contributed to some general rules for material selection with regard to their high-temperature strength and corrosion resistance.

高应变率下不同初始相变温度NiTi合金的力学响应

为获得高应变率下不同初始相变温度NiTi合金的屈服应力等基本物理特性和力学响应规律,采用10^(−3)s^(−1)应变率下准静态压缩与拉伸、10^(5)s^(−1)应变率下准等熵压缩及10^(7)s^(−1)应变率下冲击加载实现跨量级的不同应变率加载,高应变率加载实验中通过控制样品初始温度实现不同初始相态NiTi合金的力学响应测量。结果显示,初始马氏体相和初始奥氏体相NiTi合金的准静态加载应力-应变曲线中均出现2次模量变化,初始马氏体相中的模量变化由晶体重定向和马氏体相塑性变形引起,初始奥氏体相中的模量变化由马氏体相变和相变后塑性变形引起。准等熵加载下,初始马氏体相NiTi合金的Lagrangian声速随粒子速度增大而增大,未观察到间断等非线性变化;而初始奥氏体相中声速曲线存在间断,声速由初始横波值间断减小至体波声速后再随粒子速度线性增大。冲击实验中,初始马氏体相NiTi合金后自由面速度约34 m/s处出现双波结构,而将样品初始温度升至402 K后再冲击加载,则在约100 m/s处出现双波结构,二者速度曲线拐点分别由马氏体相弹塑性屈服和奥氏体相塑性屈服引起;在初始奥氏体相NiTi合金冲击实验中,在样品后自由面速度达到220~260 m/s时才出现显著的奥氏体相弹塑性转变。随着应变率从约105 s^(−1)升高至10^(7) s^(−1),相同组分奥氏体相NiTi合金的弹性极限由约2 GPa增大至约4 GPa,10^(7) s^(−1)应变率下,随着初始样品温度升至402 K,弹性极限降至1.7 GPa,表明NiTi合金的弹性极限存在显著的温度和应变率效应。

R26高温合金螺栓断裂原因

某机组汽轮机高压导气管法兰固定用R26高温合金螺栓发生断裂现象。采用宏观观察、化学成分分析、金相检验、力学性能测试、扫描电镜及能谱分析等方法对螺栓断裂的原因进行分析。结果表明:螺栓原材料中Ti元素含量偏高,使材料中产生点状夹杂析出物,经热轧加工后,形成了带状组织,造成了螺栓的高温性能下降、硬度变大,螺栓长期在高温环境下服役,最终发生了脆性断裂。

新型耐热合金C-HRA-3高温时效过程析出相成分特征分析

针对C-HRA-3耐热合金在700℃和750℃时效后的试样研究了M_(23)C_(6)碳化物和γ′相化学相组成变化规律。结果表明:C-HRA-3合金经高温时效后析出M_(23)C_(6)碳化物和γ′相,其中M_(23)C_(6)碳化物中合金元素M为Cr、Mo、Ni和Co,γ′相中合金元素有Ni、Co、Al、Ti、Mo、Cr和Nb,时效至8000 h后,700℃时效后M_(23)C_(6)碳化物析出量约0.978%,γ′相析出量约9.5%;750℃时效后,M_(23)C_(6)碳化物析出量约1.023%,γ′相析出量约6.753%,随时效温度升高,M_(23)C_(6)碳化物析出量增加约4.6%,γ′相析出量减小约28.9%。

Laves相合金的力学性能

介绍了几种重要Laves相合金的力学性能及目前的研究水平,内容包括硬度、强度、延性、力学行为和断裂韧性。提出了Laves相合金的进一步研究方向。

低温退火温度对Laves相Cr_2Nb固相热反应合成的影响

研究了退火时间为3h时,退火温度对Cr-Nb机械合金化(MA)粉的Laves相Cr2Nb固相热反应合成的影响规律,获得了30hMA粉在退火时间为3h时能使Laves相Cr2Nb固相热反应合成充分进行的最低退火温度。优化出的Cr2Nb固相热反应合成低温退火温度,可为通过MA+热压(或烧结)工艺路线制备具有微/纳米晶结构的高强高韧Cr2Nb合金或Cr2Nb基复合材料提供理论指导。

Laves相NbCr_2高温抗氧化性研究的进展

Laves相NbCr2的高温抗氧化性能限制了其作为高温结构材料在1200℃以上的应用进程.本文对Laves相NbCr2高温抗氧化性的研究进展进行了综合评述,着重介绍了Laves相NbCr2高温抗氧化性的影响因素及提高其抗氧化性的几种方法,包括合金化、稀土元素效应及防护涂层等,并就目前研究进展中的不足以及今后发展方向提出了一些看法.